This invention relates to the securing of anchor bolts in drilled openings in a mine roof in order to support the latter.
For many years, mine roof anchors of the expansion shell type have been widely used for supporting mine roofs. These anchors generally include a wedge member or nut which is threadedly engaged with the upper end of the bolt and shell segments which surround the bolt are connected by an inverted U-shape bail member, the ends of which are secured to diametrically opposed portions of the shell segments. The intermediate portion of the U-shape bail member engages the wedge member and prevents relative rotation thereof, the intermediate portion also lying within the path of the bolt to limit the longitudinal movement thereof relative to the expansion shell. Therefore, upon rotation of the bolt, the threaded engagement of the bolt and wedge member causes the latter to move longitudinally of the bolt and axially through the expansion shell, thereby causing the shell segments to expand into gripping engagement with the mine roof.
It is essential to the installation of the conventional mine roof bolt assembly that the expansion shell, bail member and wedge member remain stationary when the bolt is rotated, and this necessitates frictional engagement of the expansion shell with that portion of the mine roof defining the mine roof opening. However, in some instances, slippage occurs resulting in at least some rotational movement of the expansion shell, bail member and wedge member, thereby prolonging the installation of the mine roof anchor assembly.